Method and electrolyte for producing a copper plated microstrip

ABSTRACT

A MICROSTRIP AND METHOD OF MAKING IT, INCLUDING A SPECIALLY FORMULATED COPPER-PLATING BATH, ARE DISCLOSED. THE MICROSTRIPS ARE FABRICATED BY CHROME-COPPER EVAPORATION ON SUBSTANTIALLY PURE ALUMINUM OXIDE SUBSTRATES. AFTER CHROME-COPPER EVAPORATION, A COPPER PLATING IS APPLIED EMPLOYING THE NEW ELECTROPLATING BATH FORMULATION.

United States Patent Oflice 3,838,025 Patented Sept. 24, 1974 ABSTRACT OF THE DISCLOSURE A microstrip and method of making it, including a specially formulated copper-plating bath, are disclosed. The microstrips are fabricated by chrome-copper evaporation on substantially pure aluminum oxide substrates. After chrome-copper evaporation, a copper plating is applied, employing the new electroplating bath formulation.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to microstrips and, more particularly, to a method of producing the microstrips from a specially formulated copper-plating bath. Microstrips are employed to reduce the size of microwave hardware. In general, the microstrips serve as branch line couplers that provide the proper impedance value in non-ideal T-junctions.

2. Description of the Prior Art The fabrication of the microstrips involves a chromecopper evaporation on substantially pure aluminum oxide substrates, followed by copper-plating to provide proper skin-depth thickness. Since it could critically affect the microstrips operation, the resulting copperplating has to meet certain conditions. Grain structure must be very fine and equiaxed. Grain sizes must be consistent, and plating purity must exceed 99.9%. The plating itself must have a low stress, and the surface finish must approximate 4 microinches or better, the same surface finish required on the crome-copper substrates. The copper itself must adhere well; but, unfortunately, the presence of chrome in the substrate sometimes works against good adhesion.

Acidic copper-plating baths have been used in the past. However, the standard fluoborate and sulfate baths do not produce the fine, equiaxed grain structure required for microstrips; and the plating produced in such baths will not adhere well to the chrome-copper substrate. Moreover, acid baths normally do-not work well without organic addition agents; and since the plating purity of the instant rnicrostrips must exceed 99.9%, organic leveling agents for brightening and refining the copper-plating simply cannot be employed. Cyanide baths produce too highly stressed and rough a surface, and pyrophosphate baths do not always result in an equiaxed or consistently adherent plating.

SUMMARY OF THE INVENTION A specially formulated copper-plating bath to produce a microstrip having the desired qualities recited above, consisting of copper sulfate, sulfuric acid, sodium chloride and copper ammonium sulfate, has been discovered. Copper from the bath is electrolytically deposited upon the chrome-copper substrate which serves as the cathode in the plating bath.

OBJECTS OF THE INVENTION An object of the present invention is the provision of a very fine, equiaxed grain structure in the copper-plating on a microstrip.

Another object is to insure grain sizes within the plating are consistent.

A further object of the invention is to produce a plating purity exceeding 99.9% and a 4-microinch RMS surface finish or better.

Still another object is to increase the adherent quality of copper-plating applied to a chrome-copper substrate.

Other objects, advantages, and novel features of the present invention will become apparent from the following detailed description of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the invention, it has been found that a specially formulated copper-plating bath may be employed to produce microstrips. The fabrication consists, first, of a chrome-copper evaporation on a substantially pure aluminum oxide substrate with a 4-micro-inch RMS surface finish. Preferably, the aluminum oxide substrate is at least 99.98% pure. After chrome-copper evaporation, the resulting chrome-copper substrate is placed in the specially formulated copper-plating bath.

In electroplating, the cleaned article to be plated is connected as the cathode in a solution known as the electrolyte. Direct current is introduced through the anode and metal from the solution deposited on the cathode. In the instant invention the formulation of the copper-plating bath determines the quality of the deposition.

In its broader aspects, the invention may be carried out in an aqueous bath having ingredients in the following ranges:

copper sulfate 175 to 275 grams/liter;

sulfuric acid 32 to 100 cc./ liter;

sodium chloride 50 to grams/liter; and

copper ammonium sulfate 150 to 300 grams/ liter In the preferred embodiment, the copper sulfate concentration is 225 grams/ liter and the latter three constituents of the bath are present in exactly their minimum amounts.

Apparently, by combining two anions in a one-to-one ratio, such as copper sulfate and copper fluoroborate, it is possible to control surface roughness and copper structure without introducing organic or inorganic additives, which would eliminate the possibility of forming microstrips of the required purity for consistent electrical conductivity in plated films. The addition of sodium chloride in a relatively large amount was found to improve adhesion and lower stress. Microstrip yields were increased from 10% to after adoption of the specially formulated copper-plating bath.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appened claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An electroplating bath for producing a copper-plated microstrip, said bath consisting of 175 to 275 gm./liter of CuSO 32 to cc./liter of H 80 about 50 to 75 gm./ liter of NaCl; and

about to 300 gm./liter of Cu(NH (SO 2. The bath of claim 1 wherein the compounds are References Cited present in the following amounts: I p TENT 225 gm./liter of CuSO UN TED STATES A S 32 to (KL/liter f H2804; Greenspan R 50 to 75 gm/liter of Nacl; and r 2,445,372 7/1948 Trenbath 20452 R 150 to 300 gm mter of CMNH (so 0 2,742,412 4/1956 Cransberg et a1. 20452 R 4 2 4 z- 3 The b 2,455,554 12/1948 Brown 20452 R ath of claim 2 wherein the compounds are 3 220 897 11/1965 C l t l 204 52 R X present in the following amounts: on ey e a 225 a of @1504; FREDERICK c. EDMUNDSON, Primary Examiner 32 cc./l1ter of H 50 10 50 gm./liter of NaCl; and U.S. Cl. X.R.

150 gm./liter 0f Cu(NH (SO 20430 

